This invention relates to a composite sheet and a roof structural material comprising a wide profiled steel sheet to which is bonded an insulating layer comprising an expanded mineral core layer bonded with an inorganic binder or a mixture of an inorganic binder and a small amount of organic binder and reinforced with a thin mineral fiber reinforced covering layer covering at least one of said insulating layers.
A roof structural material requires geometric, static and physical structural properties to provide enclosure of a space, stability against deformation, supportability, insulation from heat and sound and protection from fire. The conventionally used materials such as steel, aluminum, reinforced concrete and wood provide the load supporting function and also enclose a space when assembled on a plane, but the special physical requirements for roofing can be obtained only by the use of additional insulating materials or components.